The lack of native ecclesiastical sources, such as Episcopal registers or ordination rolls, has seriously impeded research on the medieval Scottish church and its clergy. To some extent, supplications preserved in the Vatican archives have helped to overcome this problem. However, Scottish historians have yet to analyse supplications preserved in the registers of the Papal Penitentiary, one of the most important offices of the medieval papal curia. This thesis is the first study to analyse in detail a selection of Scottish supplications that were submitted to this office. It has concentrated on 1,597 supplications that were submitted by Scottish clergy to the Papal Penitentiary between 1449 and 1542. It has shown that supplicants, for the most part, approached the Penitentiary with specific issues relating to their careers. The vast majority applied for grace from impediments and/or violations of canon law in order to enter, or remain in, an ecclesiastical career. Scottish clerical supplications in the Penitentiary registers constitute a significant addition to the sources available for the study of the later medieval church in Scotland. The absence of native sources has hampered scholarly efforts to address issues such as clerical recruitment, training and ordination. Supplications preserved in the Penitentiary registers offer insight into these issues. They also serve to highlight contacts between Scotland and the papacy. Finally, they demonstrate the reception of canon law, and its application in practice, in the later medieval kingdom.